1. Field of the Invention
This invention relates to the field of continually variable transmissions that produce stepless speed ratio changes. More particularly, the invention pertains to such transmissions having a fixed speed ratio mechanism and a variable speed ratio mechanism.
2. Description of the Prior Art
Automatic transmissions for automotive use that employ a belt drive variable speed ratio mechanism usually include a torque converter located in the driveline between the power source and a variable drive mechanism, or between a forward and reverse drive gear set and the variable drive mechanism. At the lower speed ratios, the torque converter produces torque multiplication to assist in accelerating the vehicle from rest. At higher speed ratios, the torque converter is locked up to produce a mechanical connection between its input and output rather than the conventional hydrodynamic connection to avoid the losses inherent in the operation of a torque converter. At the higher speed ratios, the belt drive mechanism alone produces the torque multiplication and speed reduction between the power source and drive wheels.
It is preferred that the added expense of a torque converter and the inherent high dynamic losses associated with its operation be avoided in a continually variable transmission by using in place of the torque converter a simple fluid coupling having the capacity to attenuate driveline vibrations and harshness. However, an inherent disadvantage in merely substituting a fluid coupling for a torque converter is the loss of torque multiplication with the torque converter.
A compound planetary gear set can be operated in a regeneration mode to produce torque multiplication at lower speed ratios when its sun gear and ring gear are driven and a planet carrier is used as the output of the gear set. A compound planetary gear set can be operated in a geared neutral condition if the speed of the sun gear is increased in relation to the speed of its ring gear so that the overall speed ratio of the gear set is lowered from the speed ratio that results when the ring gear and sun gear are driven at the same speed. Still further increases of the speed of the sun gear in relation to the speed of the ring gear will produce a speed ratio that passes through zero and becomes negative when the relative speed of the sun gear increases further yet. The gearset has been operated in this manner to Produce reverse drive and forward drive. However, in certain conditions near the zero speed ratio condition, the torques carried by the mechanisms that drive the gearset inputs can rise to enormous levels, in excess of their ability to carry the load.